Defcon makes some very valid points, and you have to remember that this was not an ordinary plane crash, he would have literally glanced off the lawn.
If he had been doing the same manoever in an open space he would have slid for hundreds of yards at such a shallow angle.
When you look at the image used on the (very funny) site:
You will notice the lack of any skidding - this means one of two things:
1) It did slide along and didn't cause significant damage to the grass - proving the point, though not the one the website is trying to.
2) It hit a very steep angle (like verticle) and actually caused the same amount of damage to the ground as was seen to the pentagon wall.
No one is saying the plane hit hard and a lot of reports seem to state that more specifically, an engine struck the helipad.
That lawn is probably a thin, hard layer of topsoil ontop of hardcore. It's a lot tougher than it looks I imagine.
If the plane had plowed into it head on and it looked like that I would probably raise an eyebrow though, but it didn't.